The invention relates to a hygienic self-priming centrifugal pump with a pump housing provided with an inlet opening and a pressure connecting piece, where an impeller wheel can rotate inside said pump housing, and where said inlet opening is connected to an axial liquid ring pump section comprising an auxiliary housing and a pump screw, said pump screw being mounted on and in extension of the hub of the impeller wheel.
It has been known for a long time that centrifugal pumps are encumbered with the significant draw-back that they cannot tolerate water containing a high amount of air or gas when they are used for pumping water or other easy-flowing liquids. As soon as the amount of air pumped together with the water exceeds a predetermined percentage, typically 4 to 5%, the pump ceases to operate, which is also the case when the centrifugal pump is to be started. Then it is necessary to prime the pump, viz. to fill it with water before it is started.
The centrifugal pumps can be structured so as to overcome the above priming problems by being structured like a so-called self-priming pump. The two most conventional ways of obtaining a self-priming is 1) to separate the air from the water by said air leaving the impeller wheel and by the water being returned to a reservoir located in front of the pump; and 2) to include a vacuum pump in the centrifugal pump, such as either a positive displacement pump or a pump of the liquid ring type, where the vacuum pump is mounted on the shaft about which the impeller wheel of the centrifugal pump rotates.
When the pump is to be hygienic, i.e. when it is to be used for pumping articles of food and pharmaceutical products, said pump must be able to tolerate the cleaning and sterilizing liquids used in connection with the production of such articles, and the pump must be easy to clean. Therefore, the parts of many of these pumps must be easy to disassemble so as to allow access to the interior of the pumps in order to carry out a manual cleaning. A frequent requirement presented to the pumps is that they must be particularly easy to clean, i.e. it must be possible to clean the pump merely by circulating rinsing and cleaning liquid through said pump. Therefore, very strict requirements are presented to the structure of the pump, especially to the interior of the pump and to the surfaces and channels thereof which are in contact with the liquid. These surfaces and channels must be smooth and without crevices. The known self-priming pumps are very difficult to clean in situ in accordance with the prescribed requirements, and therefore they cannot be considered CIP-easy pumps, viz. pumps which are easy to clean in place (CIP).
From a producer""s point of view a further draw-back is found in the fact that the known self-priming centrifugal pumps are very different from the conventional centrifugal pumps because very few parts are identical in the two types of pumps. In addition, the efficiency of the known self-priming centrifugal pumps is not completely satisfactory.
The object of the invention is to provide a pump of the above type which presents a very simple structure, which in an easy manner allows both a CIP-cleaning and an ordinary manual cleaning, and which presents a higher efficiency than the known self-priming centrifugal pumps.
The centrifugal pump according to the invention is characterised in that a recycling pipe for liquid is arranged so as to connect the pressure connecting piece of the centrifugal pump on one side with the auxiliary housing of the liquid ring pump section and optionally the inlet nozzle or inlet pipe of the latter on the other side, and that all the inner surfaces in the pump housing and the auxiliary housing in direct contact with the liquid are smooth and rounded and without holes, crevices, beads or burrs.
These features render it possible for the centrifugal pump to maintain a higher efficiency than the known self-priming pumps by means of the liquid ring pump section. The recycling pipe has the effect that the risk has been avoided of the liquid ring disappearing partially or completely from the auxiliary housing while only involving an insignificant drop in the efficiency. In addition it is obtained that the centrifugal pump is suited for both CIP-cleaning and manual cleaning, and furthermore it presents a very simple structure.
According to the invention the connection of the recycling pipe to the auxiliary housing of the liquid ring pump section or to the inlet nozzle thereof may be radially arranged relative to one of these parts. As a result, the manufacture of the self-priming centrifugal pump is particularly easy.
Moreover, the connection of the recycling pipe to the auxiliary housing of the liquid ring pump section or to the inlet nozzle thereof may according to the invention be such that said connection extends tangential to one of these parts, whereby the recycled liquid is absorbed in the liquid ring instead of flowing straight through the auxiliary housing.
According to the invention the recycling pipe may optionally by means of a nozzle built therein be dimensioned such that the amount of liquid being recycled per time unit is maximum 10% of the amount of liquid passing the impeller wheel per time unit, said nozzle for instance being built into the connection between the recycling pipe and the pressure connecting piece. As a result, a particularly high efficiency is obtained for the centrifugal pump.
According to the invention an adjustable throttle valve may be built in the recycling pipe with the result that a particularly good possibility of controlling the recycled quantity of liquid is obtained.
In addition, the adjustable throttle valve may according to the invention be adapted to be automatically controlled by means of a pressure sensor arranged at the pressure connecting piece of the pump housing and/or in the auxiliary housing or in the inlet nozzle thereof, or by means of a flowmeter or a device measuring the motor output, or by means of a pressure sensor or a flowmeter arranged in the piping to and from the centrifugal pump, or by means of a differential pressure sensor measuring the pressure drop across the pump. This has proved particularly advantageous.
Moreover, the recycling pipe may according to the invention comprise short pipe stubs provided with a mounting flange, as well as an intermediary length of pipe arranged between said pipe stubs and provided with a mounting flange at both ends with the result that a particularly easy mounting of the recycling pipe is obtained.
According to the invention the recycling pipe may be adapted to be easily demountable from openings in the walls of the pump, such as holes in sleeves or sleeve stubs in said walls, and be replaced by plugs provided with a sealing rubber O-ring placed in such a manner that CIP-liquid can enter easily. As a result, when a user gradually finds out that the recycling pipe is not necessary, then he can easily remove said pipe.
When the diffuser of the centrifugal pump extends substantially vertically upwards and said pressure connecting piece is placed at the top, then the inlet pipe mounted on the inlet nozzle of the liquid ring pump section may over a portion of its length according to the invention extend substantially parallel to said diffuser and at a relatively short distance therefrom, whereby a corresponding recycling pipe may extend between the diffuser and said portion of the inlet pipe extending substantially in parallel. This has proved particularly advantageous.
Furthermore according to the invention, the outer side of the wall in the housing of the centrifugal pump and the inner side of the front wall of the auxiliary housing may be provided with a trough-shaped recess with rounded transitions extending along at least 180xc2x0 about the pump screw, said outer side of the wall in the housing including the above suction nozzle. As a result, an increase of the efficiency of the pump is obtained because vortices can easily slide off at these recesses.
According to the invention, the auxiliary housing of the liquid ring pump section may be connected to the housing of the centrifugal pump by way of welding, whereby it is possible to obtain a particularly smooth transition between the two housings and consequently it is easy to carry out a cleaning.
Finally, the connection of the recycling pipe to the auxiliary housing may according to the invention be provided by said recycling pipe extending through the front wall of said auxiliary housing, preferably obliquely through said front wall of said auxiliary housing in such a manner that at the front wall the pipe is directed towards the pump screw. This has proved particularly advantageous.